The present invention relates generally to systems and methods for combustion flame diagnostics, and more particularly to a novel non-perturbing acoustic system and method for flame temperature measurement.
The availability of tunable, high-peak-power laser sources has recently promoted the development of new techniques for combustion diagnostics relative to the identification of basic fluid and chemical properties of combustion. Techniques such as Raman, nonlinear Raman and laser induced fluorescence have been applied to thermometry of reacting flow systems. The extension of these techniques to high frequencies has suffered from the lack of high-repetition-rate, high-peak-power laser sources. High frequency thermometry has, to date, been carried out using fine-wire thermocouples or Rayleigh scattering. Both methods are extremely difficult to apply to a practical flame system.
An optoacoustic technique applied by prior workers in the field (W. Zapka et al, "Noncontact Optoacoustic Monitoring of Flame Temperature Profiles", Opt Lett 7, 477 (1982)) involved determination of flame temperature by measurement of the velocity of an intense sound pulse from a plasma spark created by focusing an intense laser beam (Nd:YAG, 1064 nm, 10-ns) on the flame to effect gas breakdown. Two spaced laser probe beams monitored the speed of the sound pulse over a distance defined by the spacing between the probe beams. The time between deflections of the two beams corresponds to the difference in arrival time of the pulse. After correction for gas flow velocity in the flame, the acoustic velocity and coresponding temperature were determined. The spark and blast wave generated by gas breakdown within the flame of the Zapka et al method require a high power laser, thereby limiting choice of excitation source, and substantially disrupts the flow within the flame, resulting in ignition of unburned fuel and air mixtures in the gases and consequent uncertainties or errors in flame temperature measurements.
The present invention overcomes disadvantages in the techniques of the prior art by providing a non-contact optoacoustic laser deflection thermometric method and system which causes no significant perturbation of flame medium during temperature measurement, while allowing localized temperature measurements within practical combustion flame environments at a repetition rate greater than the turbulence frequency of the flame. A small wire is disposed within the flame and pulsed with a low power laser pump beam, the absorption of which by the wire produces localized heating of the wire which heats the immediately surrounding gases of the flame, resulting in a pressure increase and acoustic pulse within the flame. The acoustic pulse travels outwardly in the flame, resulting in a change in the refractive index of the medium of the flame. Two parallel laser probe beams are directed through the flame between which the propagation velocity of the pulse is measured which provides a measure of the temperature of the combustion gas in the flame.
The acoustic wave generation process temperature measurement method of the present invention is non-intrusive, is substantially independent of wavelength, and high repetition measurement rates (5 kHz or more) may be achieved using commercially available lasers, detectors and data acquisition equipment.
It is, therefore, a principal object of the present invention to provide novel non-perturbing acoustic method and system for flame temperature measurement.
It is a further object of the invention to provide a non-intrusive method and system for accurate and repetitive high speed temperature measurement of combustion flames.
These and other objects of the invention will become apparent as the detailed description of representative embodiments thereof proceeds.